This invention relates to a method for the production of an optical phase-shifting board and a method for the fixation of the optical-shifting board to a semiconductor laser stem.
2. Description of the prior art:
Optical phase-shifting boards by which optical phase-shift between the laser beams emitted from a semiconductor laser array device can be attained are essential to the construction of an optical integrated circuit. FIG. 5 shows a conventional phase-shifting board attaining a 180.degree. phase-shift, which is produced as follows: A glass substrate 201 is subjected to a photolithographic treatment, resulting in a striped pattern. The width W' of each stripe is 5 .mu.m and the stripes have a pitch of 2 W' (i.e., 10 .mu.m). The portions of the surface of the glass substrate which is not to be etched are coated wtih a photoresist. The glass substrate with such a striped pattern is then subjected to an etching treatment with an etchant that is a mixture of hydrofluoric acid, ammonium fluoride and water, resulting in a mesastriped pattern therein wherein the depth d' of the etched portions is set to meet the equation (1). ##EQU1## wherein n' is the refractive index of the glass substrate, m is an integer such as 0, 1, 2, 3, . . . , and .lambda. is the wavelength of light in the air.
Then, the photoresist is removed from the glass substrate, resulting in an optical phase-shifting board.
The quantitative control of the phase-shift by the optical phase-shifting board obtained by the above-mentioned method depends upon the depth d' of the etched portions of the glass substrate, and the scatter of the quantities of the phase-shift corresponds to that of the depth d'. It is extremely difficult to achieve a uniform etching process in the plane and to improve the reproducibility of the etchng process. Moreover, the etching process is complicated and needs skilled workers.
Another conventional phase-shifting board can be made by a method in which the refractive index of a glass substrate is changed using an ion-exchanging technique, but the depth of each striped groove that is formed by the ion-exchanging technique cannot be controlled.
On the other hand, semiconductor laser devices are widely used in optical measuring systems, optical communication systems, etc., in which laser beams from the semiconductor laser devices are formed into useful shapes by means of lenses and/or prisms. Especially, in order that the optical phase-shift between the adjacent laser beams just emitted from a semiconductor laser device is achieved, a near-field pattern must be formed using an objective lens and a phase-shifting board must be disposed at the position where the near-field pattern is formed as reported by, for example, J. R. Heidel et al., IEEE Journal of Quantum Electonis, Vol. QE-22, No. 6, pp 740-752 (1986). FIG. 6 shows the optical system that is constituted by the above-mentioned method, in which a laser beam emitted from a semiconductor laser device 1 converges on the optical axis of a lens 2 via the lens 2, and an optical phase-shifting board 3 is disposed at the point of convergence. However, this system is large and the optical elements constituting this optical system must be positioned with accuracy so as to align their optical axes. Moreover, the positional relationship among the semiconductor laser device 1, the lens 2 and the phase-shifting board 3 must be fixed, which causes difficulties in the miniaturization of this system and the stabilization of the performance of this system for a long period of time.